Once William, Duke of Normandy, had succeeded in vanquishing the English at Hastings, he set out to tighten his hold on the rest of the country. Reasonable thought. Instead of marching straight to London, he took time to capture the major southern ports. Dover surrendered without a fight.
While at Dover, a large number of William’s army went down with gastroenteritis. William also fell ill and the mobile portion of his army camped at Canterbury (that, incidentally, is a city you should put on your bucket list. It’s crawling with history and literature and the cathedral where Becket was “martyred” – T.S. Elliot “Murder in the Cathedral” – is well worth the visit.)
Thus do sanitation and sanitary practices rear their ugly heads.